It is well known that aerodynamic testing of a model, or an article, installed in a wind tunnel test section never truly represents the flow conditions of flight, i.e., the test article in an unbounded fluid stream. The boundaries of the test stream (the flow conditions at the wind tunnel walls) in the presence of a test model, such as a body, lifting wing, or wing-body combination produce extraneous velocity components in the fluid flow around the model. These wind tunnel wall interferences in the test section perturb the fluid flowing past the model requiting test data corrections. At least four generations of experimenters have routinely corrected both the magnitude and direction of the nominal onset air stream, resulting in modifications to the measured aerodynamic characteristics of the model (lift, drag, and pitching moment as a function of angle of attack). See, for example, "Low Speed Wind Tunnel Testing" by A. Pope, J. J. Harper (John Wiley Sons, Inc. 1966).
A number of proposals for simulating interference-free conditions or for minimizing wall interference effects within wind tunnels test sections have been proposed. See "Self Correcting Wind Tunnels" by W. R. Sears, Aeronautical Journal, February/March 1974, page 80. For example, it is well-known to use adaptive wall wind tunnels wherein deformable flexible solid test section walls form a substantially interference-free streamtube. In the ideal solution, arbitrarily deformable walls would provide; a complete adaptation, with the interference velocity field being identically zero everywhere in the vicinity of the model.
For airfoil, or so-called two-dimensional, testing, wherein a wing of constant cross-section spans a wind tunnel from one sidewall to the opposite sidewall, this can be achieved by deforming the floor and ceiling in simple streamwise curvature. This is readily achieved using flexible walls and a multi-jack wall positioning system. For arbitrary three-dimensional models (airplanes, non-lifting bodies, swept or tapered wings, etc.) this strategy can be employed to approximate an ideal interference-free streamtube by providing for wall deformation in streamwise strips, as exemplified by U.S. Pat. No. 4,308,748 issued to Jacocks, U.S. Pat. No. 5,046,358 issued to Wulf, et al., European Patent Applications 27,229A1, 365,799A2, and 572,787A1 of Wedemeyer, Amecke, et al. and Bouis, respectively.
Another approach for an adaptive wall wind tunnel includes the use of walls with controlled crossflow. In other words, instead of employing solid, or impermeable, walls, openings in the test section walls are provided to allow air to pass from the test section airstream into art external volume (i.e., a plenum) of nominally stagnant air, and back again depending on the pressure imposed by the model on the walls. Non-adaptive ventilated walls have been found to lessen wall interference, compared to closed wall non-adaptive tunnels, for an appropriate choice of global ventilation (meaning that wall openness is uniform throughout the test section). See Goethert, B. "Transonic Wind Tunnel Testing", AGARDograph 49, Pergamon Press, 1961.
There are two main types of ventilated walls in common usage: perforated and slotted. Perforated walls have a uniform pattern of holes, drilled either normal to the wall surface, or at an angle of up to 60 degrees to the normal. Control of wall openness can be effected by providing a sliding or translatable perforated backing plate whose position determines the degree of wall openness, or porosity. This concept has been implemented in the AEDC 1T Transonic Wind Tunnel for each wall separately, and in the T128 Transonic Wind Tunnel run by TsAGI in Zhukovsky, Russia, which uses 128 independently slidable perforated backing plates which can vary wall porosity of each wall segment from 0% to 10%. UK Patent Application 2,177,661A provides another means of varying the resistance of perforated walls by utilizing individual movable plugs for each hole in the wall.
As will be understood to one of ordinary skill in the art, modifying the flow through ventilated walls can be done in such a way as to approximate the interference-free flowfield (due to the model) at the walls (determined by calculation for the model in question), and thus minimize the wall interference. By controlling the amount of airflow through the walls (i.e. crossflow), for example, by throttling the wall openings to restrict fluid flow from the test section to and from the surrounding plenum, a substantially interference-free streamtube can be formed in an analogous fashion to the solid flexible wall technique. In one approach, walls with separate segmented pressure plena freely communicate with the test section through ventilated surfaces, such as slotted or perforated wall surfaces. Each plenum pressure and therefore the flow through the walls can be separately controlled. See U.K. 2,142,290A Cook et at. In another approach, the test section walls are provided with mechanical elements that can vary the wall openness over local areas of the wall independently of each other, with all wall openings venting to a common constant pressure plenum chamber. Examples of this type of wind tunnel wall include the aforementioned T128 Tunnel and United Kingdom Patent Application 2,177,661A of Heddergott, et al. and DE 3,404,696 C2 of Amecke, et al
In contrast to perforated walls which have perforations uniformly distributed over an entire wall surface, slotted walls have longitudinal slots in the walls separated by bands of impermeable material. The slots may be either open, or may have baffles, perforated inserts, or other flow resistance device within the openings. Wall interference of slotted walls is most simply modified by locally varying the width of each slot. The T1500 Transonic Wind Tunnel operated by FFA in Stockholm, Sweden, has provision for installation of slot inserts which provide the capability for arbitrary, but fixed, slot widths. There is no provision for automatic variability of slot width.
The present invention provides the means for adjusting the crossflow resistance of ventilated walls (either slotted or perforated) by utilizing a flexible throttle plate over each section of wall to be controlled. It has the advantage of providing a continuous variation of crossflow resistance in the streamwise direction, and thereby the ability for complete wall adaptation for interference-free flow, rather than being limited to a piecewise discontinuous approximation to the desired smooth variation of wall properties provided by a segmented variable porosity solution as in the Russian T128 Transonic Wind Tunnel.
The present invention has the advantage of fewer actuators, moving parts, and seals, providing a relatively simply and ostensibly more dependable operation than other controllable ventilated wall concepts, such as illustrated in UK patent Application 2177661A which requires a plug for each hole.
Adaptive ventilated wall tunnels with separate controlled plena have the disadvantage of requiring a complex pressure and vacuum air plant for plenum pressure control, rather than relying on the relatively simple mechanical actuation of the present invention.
The use of solid flexible walls for adaptive wall wind tunnels has been largely limited to two-dimensional testing, or to only partial adaptation due to the practical difficulty of bending a wall simultaneously in two directions, both longitudinally (streamwise) and laterally (cross-stream), which would be required for complete adaptation for three-dimensional models. This invention provides for lateral control variability by allowing adjacent throttle plates to be deflected independently.